Method and Device for Precision Machining of Toothed and Hardened Work Wheels

ABSTRACT

A method for machining toothed and hardened work wheels, includes: mounting a work wheel that is hardened and pre-toothed with an allowance onto a workpiece spindle; removing at least 50% of the allowance by means of gear skiving with a skiving wheel that is rotatably driven by a tool spindle; precision-machining the work wheel in unchanged tension by means of a honing wheel. The forward movement occurs during gear skiving in the extension direction of the toothing. The delivery of the workpiece that is moved in an oscillating manner in the extension direction of the toothing occurs during honing in the radial direction. The skiving wheel and the honing wheel are driven by a common tool spindle. A device for carrying out the method includes a workpiece spindle, which is driven to rotate, and a tool spindle, which carries a combination tool having a skiving wheel and a honing wheel.

TECHNICAL FIELD

The invention relates to a method and a device for machining toothed andhardened work wheels as well as a tool which is usable in this regard.

BACKGROUND

Hardened gearwheels are manufactured by a plurality of successiveoperations. First, a blank made of unhardened steel is provided withinternal or external toothing by one of the many methods known from thestate of the art. This can take place by ramming, gear hobbing orgear-skiving. The pre-toothed work wheel is hardened next. After beinghardened, precision machining is performed by honing/grinding. Precisionmachining is carried out using a plurality of machine tools that aredifferent from one another, such that the work wheel is rechucked eachtime. This requires keying of the honing wheel into the tooth gaps onthe toothed work wheel, for example, because the latter is chucked witha concentricity error. In precision machining, an allowance retainedpreviously is removed from the teeth of the toothing to be fabricated.The allowance is distributed irregularly over the circumference of thework wheel due to an unavoidable chucking error and unavoidablehardening distortion.

DE 35 33 064 A1 describes a method for machining the flanks ofgearwheels by gear skiving as well as a suitable device for doing so.The flanks of unhardened gearwheels, but also hardened gearwheels withstraight or helical toothing, should be machined by gear skiving.Grinding or hard shaving, but also honing or precision grinding, shouldbe replaced by gear skiving of hardened tooth flanks.

DE 103 05 752 A1 describes a combination tool for toothing ofcylindrical components, wherein a skiving wheel for gear skiving and atleast one shaving cutter for shaving are arranged on a common shaft sothat in two successive machining steps, gear skiving can be carried outon one machine first and then shaving can be carried out without aworkpiece change.

SUMMARY

The object of the invention is to improve on the manufacturing methodfor toothing of hardened work wheels and to provide a device in thisregard and/or a tool usable for this purpose.

This object is achieved by the invention as defined in the claims. Thedependent claims not only constitute advantageous refinements of theindependent claims but are also independent solutions to the problem.

First and essentially, it is proposed that the pre-toothed and hardenedwork wheel, wherein the toothing has an allowance, is chucked on aworkpiece spindle. Most of the allowance, i.e., at least 50%, preferablymore than 80% or more than 90% of the allowance, is removed during thischucking by gear skiving using a skiving wheel driven to rotate by atool spindle. The allowance is greater than the hardness error and/orthe chucking error, so that no air cuts are carried out in gear skiving.There is no keying to the allowance, which still remains unevenlydistributed due to the process. In a subsequent precision machiningstep, in which the work wheel remains chucked on the workpiece spindle,the tooth flanks are subjected to precision machining by means of ahoning wheel. Due to the preliminary gear skiving process, all roundingerrors and hardness defects have been compensated, so that the honingwheel need not be keyed into the toothing of the work wheel. In apreferred embodiment of the method, skiving machining is carried out notin several successive steps but instead in a single cut, wherein thefeed motion takes place once, namely outside of the workpiece in theprofile direction, i.e., in the radial direction. However, the skivingmachining may also comprise several successive steps. The forwardmovement then takes place when cutting is not done in the profiledirection. In machining, the forward movement takes place in thedirection of extent of the tooth, for example, in the axial direction inthe case of straight toothing. The cut therefore advances directly intothe solid material. In this skiving machining, the hardness defects andconcentricity defects, in particular the concentricity defects caused bythe chucking, are eliminated by removing a significant amount of theallowance. However, the toothing precision is still inadequate.Furthermore, the tooth flanks of the work wheel, which has only beenhardened in gear-skiving, still does not have a sufficient surfacequality. The required surface quality and/or toothing precision is/areachieved by the honing machining step. The method according to theinvention thus comprises a hard-roughing step that removes a significantportion of the allowance, and a successive hard-smoothing step, whereinthe roughing step is carried out by gear skiving, and the smoothing stepis carried out by honing, and both of these machining steps are carriedout in the same chucking of the work wheel. The roughing step preferablytakes place using a hard metal skiving wheel, and the smoothing steppreferably takes place with a ceramically bonded honing wheel. Themachine tool has a dressing master ring with diamond-tipped teeth. Thehoning wheel may be a CBN tool. It is regarded as advantageous that thegreatest accuracy and the best surface quality can be achieved with themethod according to the invention. Due to the dressing of the honingwheel in the machine itself, negative effects of the tool due toroundness, chucking and manufacturing defects are prevented. During theforward movement in gear skiving in the direction of extent of thetoothing, i.e., in the axial direction of the workpiece spindle in thecase of straight toothing, the adjustment takes place in the profiledirection during honing, i.e., in the radial direction. In this case,the workpiece oscillates in the direction of extent of the tooth, i.e.,in the axial direction of the workpiece spindle in the case of straighttoothing. The honing wheel and the skiving wheel may be formed by acombination tool. Then the honing wheel and the skiving wheel sitaxially, one after the other, and are driven to rotate by a common toolspindle. However, it is also possible to use two tool spindles that aredifferent from one another, such that the first tool spindle carries thehoning wheel and the second tool spindle carries the skiving wheel. Thisvariant is selected in particular when the use of a combination tool isimpossible in a case of conflict. Both machining methods, i.e., skivingof a hardened work wheel and honing of the toothing, which has beenbrought to contour with a honing wheel, operate essentially by the sameprinciple and/or with the same machine kinematics. Therefore, the twomachining steps can be carried out in the same machine with adjustedrates of forward movement and cutting speeds. Each of the two methodsteps makes use of the advantage inherent in the method. The deviceaccording to the invention has a workpiece spindle that can be driven torotate and has a tool spindle that can be driven to rotate. The twospindles are each driven by electric motors that are in synchronization.An electronic control unit, which operates according to a machiningprogram, is provided. This electronic control unit controls therotational movement of the spindles, the adjusting movement and theforward movement. According to the invention, the electronic controlunit is equipped, i.e., programmed, so that the method steps describedin the introduction are carried out in the order described previously.The tool according to the invention is a combination tool. In this tool,the skiving wheel and the honing wheel are connected to one anothercoaxially. The skiving wheel may be arranged axially upstream from thehoning wheel. The skiving wheel may have a smaller diameter than thehoning wheel. The honing wheel is a dressable honing wheel, inparticular a CBN honing wheel. The dressing tool used for dressing thehoning wheel is part of the device. The work wheel is preferably ahardened pre-toothed gear wheel. The pre-toothing has the allowance andis preferably produced by gear skiving, for example, before the workwheel is hardened but is also produced by another method, which ismentioned in the introduction to the description. The hardenedpre-toothed work wheel may be made of a case-hardened steel with acarbon content of up to 0.2%. According to other specific embodiments ofthe invention, however, the steel also has a carbon content of more than0.2%. In hardening, the work wheel is heat-treated and develops agreater hardness at its surface and in the underlying structure. In apreferred variant of the invention, the pre-toothed workpiece has asurface hardness between 52 HRC and 63 HRC (hardness according toRockwell, measured with a diamond cone). In other embodiment variants,the surface hardness may also be less than 52 HRC or even less than 45HRC and/or more than 63 HRC. The hardness is achieved by a conversion ofstructure between iron and carbon during the heat treatment process.Martensite is formed and carbides are deposited. According to theinvention, the hardened pre-toothing is brought to the qualityultimately required by means of hard precision working after the heattreatment and after gear skiving. The surface hardness on the flanks ofthe teeth may have a constant value over a certain thickness, dependingon the heat treatment process and the material over a certain thickness.The invention relates to specific embodiments, in which the hardness ofthe workpiece decreases toward the interior of the tooth. The corehardness and/or core strength is then lower than the surface hardnessand/or surface strength. In some variants of the invention, the depth ofthe hardness may vary. In precision machining of the work wheel,material is removed in an amount between 10 μm and 100 μm or between 10μm and 150 μm. In one variant of the invention, it is provided that thematerial hardness does not change over the thickness of the allowance,such that the allowance may be in the range between 20 μm and 300 μm.The workpiece to be cut in this way should have a hardness of at least45 HRC. According to one variant of the invention, the pre-toothedhardened blank has a tensile strength of R_(m)=1430 N/mm². According toone variant of the invention, the pre-toothed and hardened workpiece isthoroughly hardened over at least the thickness of the allowancematerial. The tooth flanks are formed to a certain extent by a surfacezone having essentially the same hardness, which may amount to at leastmore than 45 HRC, with this zone having a depth measured perpendicularto the plane of extent of the tooth flank, said depth being greater thanthe allowance and in particular being greater than 100 μm, preferablygreater than 200 μm. Both the gear-skiving machining and the precisionmachining thereafter thus take place in the area of the hardened surfacezone, so that the surface of the tooth flanks remaining after precisionmachining has a hardness of at least 45 HRC, but this hardness may alsobe in the range of 52 HRC to 63 HRC. This method is preferably carriedout using a combination tool, with which a skiving wheel and a honingwheel are mounted on a common driveshaft, such that the skiving wheeland the honing wheel have a fixed angle-of-rotation position relative toone another. In traditional honing for correction of hardness defects,new toothing is introduced into the workpiece to some extent, whereasonly the contour deviations still present after hard skiving are wornaway in the honing machining step according to the invention. Thesedeviations from the ideal contour are essentially the peaks of surfaceelevations. The valleys are preserved in honing. According to theinvention, the main removal of material takes place during gear skivingof the hardened flanks. Due to this gear skiving, the flanks are broughtto approximately the ideal contour, so that only the peaks of theelevations created in gear skiving are leveled, i.e., worn away, by thesubsequent honing operation. Honing is used as a simple smoothingoperation, in which material is not abraded over the full area of thetooth flanks but instead is abraded only on the islands protruding abovethe ideal contour. This reduces the heat induced in the component duringmachining. The precision machining step according to the invention thustakes place preferentially and without the use of a cooling medium. Thegear skiving operation therefore also takes place without the use of acooling medium. The gear skiving process also takes place without theuse of a cooling medium. Since the honing wheel and the gear skivingwheel are at a predetermined angle of rotation to one another and areboth mounted on the same shaft, the precision machining tool can bebrought directly into the machining position after gear skiving withouthaving to make an individual correction of the angle of rotation betweenthe workpiece spindle and the tool spindle. The precision machining ofthe work wheel thus takes place not only in a different chucking bymeans of a honing wheel but also merely through a change in the settingof the tool spindle to the workpiece spindle after gear skiving,preferably without a relative rotation of the two spindle axes that goesbeyond the known difference in the angle of rotation between the skivingwheel and the honing wheel. This method is characterized in particularby the fact that only island-like regions of the tooth flanks of thework wheel are worn away in precision machining of the work wheel, andsections of the tooth flanks remaining between the islands are notmachined in precision machining.

BRIEF DESCRIPTION OF THE DRAWINGS

One exemplary embodiment of the invention explained below on the basisof the accompanying drawings, in which:

FIG. 1 shows schematically a workpiece spindle 4 with a chuck 3, whichchucks a workpiece 1, and a tool spindle 7, which carries a combinationtool comprising a skiving wheel 5 and a honing wheel 6;

FIG. 2 shows a diagram according to FIG. 1 in skiving machining; and

FIG. 3 shows a diagram according to FIG. 1 in honing machining.

DETAILED DESCRIPTION

The drawings show only details of a machine tool according to theinvention. The machine tool according to the invention has a machine bed(not shown in the drawings) and a housing. Inside the housing there is aworkpiece spindle 4, which is driven to rotate by an electric motor. Inaddition, a tool spindle 7 is also provided, with a skiving tool 5 and ahoning tool 6 mounted thereon. The skiving tool 5 is preferably a hardmetal skiving wheel, which is arranged coaxially with a honing wheel 6,preferably made of CBN. The tool spindle 7 is driven by an electricmotor. The electric motors are synchronized with one another in such away that the workpiece spindle 4 and the tool spindle 7 can be driven torotate in synchronization. In addition, drives, which permit anadjustment and a forward movement, are also provided. All the drives aredriven by an electronic control unit. The electronic control unit has aprogram memory for storage of a machining program. The machining programincludes a machining method.

A work wheel 1 is prepared to carry it out the machining method. Thework wheel 1 is first provided with teeth in an unhardened state. Theteeth have an allowance of, for example, four to eight hundredths of amillimeter, but the allowance may also amount to only six hundredths ofa millimeter. The allowance is sufficient to correct any hardnessdistortion and defects in shape and/or concentricity. Next, the workwheel, which has been pre-toothed in this way is hardened. This resultsin hardness distortion.

In a first method step, the hardened work wheel, after being pre-toothedwith an allowance, is chucked on the tool spindle 4. In a second methodstep, most of the allowance is removed by gear skiving using the skivingwheel 5 that is driven to rotate by the tool spindle 7. The machiningtakes place to a certain extent into the solid material, and in a stepin which all the tooth flanks of the toothing 2 of the work wheel 1 arebeing machined. The gear skiving machining takes place with a forwardmovement Vs in the direction of extent of the teeth. The workpiece axisand the tool axis, i.e., the axes of rotation of the tool spindle 7 andthe workpiece spindle 4, form an axle-crossing angle. There ispreferably no shift in forward movement in the radial direction duringgear skiving. The skiving wheel 1 may be manufactured from hard metal,ceramic or some other suitable hard material.

In a third method step, which is carried out without re-chucking of thework wheel 1, precision machining of the hard machined toothing 2 iscarried out by means of gear skiving. To do so, the honing wheel 6 isbrought into a machining position. During the honing machining, the workwheel 1 oscillates in the direction of extent of the teeth. The feedmotion V_(H) here is in the radial direction.

The preceding discussion serves to illustrate the inventions, which arecovered on the whole by the present patent application and which reflectthe prior art at least through the following combinations of features,even independently, namely:

A method for machining toothed and hardened work wheels 1, comprisingthe following steps:

-   -   chucking the work wheel 1, which has been pre-toothed with an        allowance and hardened, on a workpiece spindle 4;    -   removing at least 50% of the allowance by gear skiving with a        skiving wheel 5 that is driven to rotate by a tool spindle 7;        and    -   precision machining of the work wheel 1 by means of a honing        wheel 6 with no change in chucking.

A method, which is characterized in that the forward movement Vs in gearskiving takes place in the direction of extent of the toothing 2.

A method, which is characterized in that the feed motion V_(H) of thework wheel 1, which is moved by oscillation in the direction of extentof the toothing 2, is carried out in honing in the radial direction.

A method, which is characterized in that the skiving wheel 5 and thehoning wheel 6 are both driven by the same tool spindle 7.

A method, which is characterized in that the skiving wheel 5 is aceramic skiving wheel or a hard metal skiving wheel.

A method, which is characterized in that the honing wheel 6 is aceramically bonded, dressable tool, in particular a CBN tool.

A method, which is characterized in that the gear skiving machiningtakes place in a single step.

A method, which is characterized in that the honing wheel 6, which ismounted on the tool spindle 7, is dressed by means of a dressing toolassigned to the device and mounted on the workpiece spindle 4 inparticular.

A device, which is characterized in that the control unit is equipped sothat at least 50% of the allowance can be removed by gear skiving with askiving wheel driven to rotation by the tool spindle 7 after a workwheel 1, which has been pre-toothed with an allowance and hardened, hasbeen chucked on the workpiece spindle 4, and then the work wheel 1 whilein the same chucking is precision machined by means of a honing wheel 6driven to rotate by one of the tool spindles.

A device, which is characterized in that the skiving wheel 5 and ahoning wheel 6 sit on the same tool spindle 7.

A device, which is characterized by a dressing tool with which thehoning wheel 6 that is mounted on the tool spindle 7 can be dressed.

A tool, which is characterized in that a skiving wheel 5 and a honingwheel 6 are combined coaxially and axially behind one another to form acombination tool, which can be driven to rotate by the same tool spindle7.

A method, a device or a tool, which are characterized in that theskiving wheel 5 is a hard metal skiving wheel 5.

A method, a device or a tool, which are characterized by a ceramicallybonded honing wheel 6, in particular one that can be dressed and is aCBN tool in particular.

All the features disclosed here (either separately or in any combinationwith one another) are essential to the invention. The full content ofthe disclosure of the respective/attached priority documents (photocopyof the previous patent application) is herewith included in thedisclosure of the present patent application, also for the purpose ofincluding features of these documents in the claims of the presentpatent application. The dependent claims with their featurescharacterize independent refinements of the prior art according to theinvention, in particular to be able to make divisional applications onthe basis of these claims.

1.-15. (canceled)
 16. A device for carrying out a method for machining awork wheel being prepared in an unhardened state with teeth having anallowance; being hardened to a surface hardness of at least 45 HRC andhaving a hardness distortion error less than the allowance, comprising:a workpiece spindle that is configured to be driven to rotate, theworkpiece spindle having a chuck for chucking the work wheel to bemachined; a tool spindle that is configured to be driven to rotate insynchronization with the workpiece spindle, the tool spindle holding askiving wheel and a honing wheel; and a programmed electronic controlunit controlling a rotational movement of the tool spindle and that ofthe workpiece spindle, a feed motion and a forward movement; wherein thecontrol unit is equipped so that, after chucking said work wheel ontothe workpiece spindle, at least 50% of the allowance is removable bygear skiving with the skiving wheel driven to rotate by the toolspindle, and then the work wheel is configured to be precision machinedusing the honing wheel driven to rotate by the tool spindle during thesame chucking, and wherein the forward movement in gear skiving takesplace in a direction of extent of the gearing, and the feed motion ofthe work wheel oscillating in a direction of extent of the toothingtakes place in the radial direction during honing, wherein the skivingwheel and the honing wheel sit on the same tool spindle and are formedby a combination tool.
 17. The device according to claim 16,characterized by a dressing tool being located in the machine fordressing the honing wheel being carried by the tool spindle.
 18. Adevice according to claim 16, wherein the skiving wheel is a hard metalskiving wheel.
 19. A device according to claim 16, wherein the honingwheel is a ceramically bonded, in particular dressable, honing wheel.20. The device of claim 16, wherein the honing wheel is a CBN tool. 21.A tool comprising a skiving wheel and a honing wheel being combinedcoaxially and axially, one after the other, to form a combination tool,which is configured to be driven to rotate by the same tool spindle. 22.A tool according to claim 21, wherein the skiving wheel is a hard metalskiving wheel.
 23. A tool according to claim 21, wherein the honingwheel is a ceramically bonded, in particular dressable, honing wheel.24. The tool according to claim 21, wherein the honing wheel is a CBNtool.